1. Field of Invention
The present invention relates generally to power monitoring of lasing semiconductor optical amplifiers (SOA). Particularly, the present invention relates to monitoring emitted lasers through a cavity surface of the lasing SOA in order to determine a gain corresponding to the lasing SOA. More particularly, the present invention relates to monitoring optical signal power and adjusting gain across the lasing SOA in order to optimize the optical signal power.
2. Description of Related Art
Service providers are experiencing an ever-increasing demand for bandwidth fueled by Internet access, voice, data, and video transmissions and this demand will continue to grow. Due to this demand, network capacities are being stretched to their limit. As a result, there has been an increasing effort to lay fiber to build out the backbone of existing networks.
These efforts to increase the size of network infrastructures are still unable to meet the growing demand of bandwidth-hungry clients. In response, service providers are continually trying to maximize the utilization of their existing network. However, in order to efficiently utilize a network, a service provider must be able to monitor the capacity of each path within the network. Generally, in order to accomplish the requisite monitoring of networks, a service provider must employ a comprehensive array of network management components and applications. However, the cost of these network management systems is often very high. Also, often times the reliability of the network management systems is questionable due to its complexity.
A key factor in the efficient utilization of a network is the ability to detect the failure of a specific path within the network. In addition to determining whether a network path has failed, it is also important that the source of the failure can be detected and subsequently repaired. Thus, there is a need for a system and method of efficiently detecting and reporting network failure within an optical network.
In an optical communications system, maintenance is performed at the optical level. Because an optical signal contains multiple signals having differing wavelengths, a network failure may only affect certain wavelengths within the signal. Therefore, it is important that components within the network, such as optical amplifiers, permit these other wavelengths to travel within the network.
Optical amplifiers, which boost the power of optical signals, are a basic building block for many types of optical systems. For example, fiber optic communications systems transmit information optically at very high speeds over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations. As another example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers are used to boost an optical signal to the proper power range for the receiver. Thus, it is important to monitor the power of an optical signal at both the input and output of an amplifier in order to determine how much amplification to provide.
An optical amplifier is used to apply a gain to an optical signal. This gain is measured by the power of the signal leaving the amplifier divided by the power of the signal entering the signal. Therefore, if the signal""s gain through an amplifier is greater than one, then the amplifier has amplified (i.e., increased the signal""s power) the signal.
In order to ensure that signals are properly transmitted to a destination on an optical network, amplifiers within the network preferably have variable gain. For example, consider a situation in which a trunk line containing fiber amplifiers fails. If certain wavelengths can still be transmitted through the network trunk line during a failure, then fiber amplifiers within the network preferably will be adjusted to avoid applying too high a gain. One approach to controlling the gain on an optical amplifier is based on detecting both the input and output power associated with the amplifier.
One method typically used to detect the input and output power on an optical amplifier is to tap a portion of an optical signal using power couplers before it enters the optical amplifier and after it leaves. These tapped optical signal portions are diverted to optical detectors that covert them to electrical signal. These electrical signals are then analyzed to determine the signal""s power at the amplifier input and output. However, the optical signal""s strength is reduced by this method. This detection process reduces the strength of the optical signal because of these taps at the input and output of the optical amplifier. The input tap loss is approximately equal to the ratio of the power at the tap port and the power at the input port of the optical amplifier. The output tap is equal to the ratio of the power at the output and the power at the output of the tap port. Thus, the signal has a loss associated with detecting the gain across the amplifier equal to the loss of the first tap plus the loss of the second tap. As optical networks expand and the number of amplifiers increase, these tap losses may drastically effect the efficiency of the network. Therefore, there is a need for a monitoring system that is able to effectively determine power levels of an optical signal before and/or after amplification and still avoid tap losses caused by power couplers.
The present invention overcomes the deficiencies and limitations of the prior art by providing an optical signal power monitor and lasing SOA gain control system. In particular, the present invention provides an optical power detection system that avoids the use of power couplers or taps by detecting a ballast laser signal emitted from a lasing semiconductor optical amplifier (SOA). The lasing SOA emits the ballast laser signal in response to the amplification of the optical signal.
An optical signal propagates through an amplification path within the lasing SOA causing the signal to be amplified. A laser cavity within the amplification path contains a semiconductor gain medium that is pumped above a lasing threshold for the laser cavity. As a result, lasing occurs producing laser radiation within the laser cavity. This laser radiation operates as a ballast to prevent gain saturation within the laser cavity during the amplification of the optical signal. As a result, the gain within the laser cavity is clamped causing the laser radiation to be emitted as a ballast laser signal in order to avoid saturating the laser cavity gain.
This ballast laser signal corresponds to the strength of the optical signal within the laser cavity because the laser cavity gain is clamped. The present invention utilizes this correlation in order to determine a power level of an optical signal as it propagates through a lasing SOA. Specifically, detectors are placed above the substrate that emits this ballast laser signal. In an embodiment, a detector is placed above the emitting substrate near the input of the lasing SOA. A second detector is placed above the emitting substrate near the output of the lasing SOA. The first detector converts the ballast laser signal to a first electrical signal corresponding to the optical signal near the input. The second detector converts the ballast laser signal to a second electrical signal corresponding to the optical signal near the output. These two signals may be analyzed to determine the optical signal power at the input and output as well as used to calculate a gain across the lasing SOA.
In response to determining the optical signal power, the gain of a tunable-gain lasing SOA or a multi-stage lasing SOA may be adjusted to optimize the optical signal strength as it propagates through a network. A tunable-gain lasing SOA includes a tunable region in the laser cavity that allows the loss within the laser cavity to be adjusted. Because the gain of the lasing SOA is clamped, the magnitude of the laser cavity loss controls the gain within the laser cavity. As a result, the gain across the lasing SOA may be dynamically adjusted. A multi-stage lasing SOA comprises multiple lasing SOAs coupled in series and may include tunable-gain lasing SOAs. The gain across multi-stage lasing SOAs is approximately equal to the sum of the gain for each of the connected lasing SOAs. This gain may be adjusted by tuning various lasing SOAs on or off. Additionally, if tunable-gain lasing VLSOAs are used, then the multi-stage lasing SOA gain may be adjusted by controlling the gain on the tunable-gain lasing VLSOAs.
A terminal on a network may control the adjustment of a lasing SOA gain dynamically, thereby reducing the time required for a network manager to monitor the network. Additionally, this terminal may generate various warnings if an optical signal power is outside an optimal range. Finally, the gain across lasing SOAs may be easily increased in order to compensate for the insertion of additional channels within the network or a physical build-out of the network.